Method of inhibiting corrosion



ite Sttes ice 3,052,575 METHUD F INHKBITING CORROSIQN Wilhur G. Crane,Calumet City, lll., assignor to Inland Steel Company, hicago, Ill., a.corporation of Dela- Ware No Drawing. Filed Feb. 13, 1961, er. No.88,649 7 Claims. (Cl. 148-6.14)

The present invention relates generally to a method of protectivelycoating a metal surface and more particularly, to an improved method ofcoating a ferrous metal to protect the ferrous metal from corrosion onexposure to moist air and other conditions providing high concentrationof moisture.

The manufacturers of metal sheets, such as black plate of the typecommonly used for making metal containers, and manufacturers of metalcontainers and other fabricators using black plate have long beenseeking improved methods for rapidly treating metal surfaces to reduceor prevent rusting thereof due to atmospheric moisture during shipmentand storage. It has been required that the surface of the treated metalshould be capable of retaining a lacquer coating and preferably shouldalso provide under-film corrosion resistance.

The use of chemical bath-s or sprays for providing a coating on metalsurfaces to improve corrosion resistance has long been practiced. Forthis purpose, solutions of phosphates, chromates, nitrates, nitrites,tungstates, molybdates, and many other salts have been proposed. None ofthe prior art method-s, however, has been entirely satisfactory,particularly where the period of treatment must be extremely brief, aswhen the treatment is carried out continuously on a high-speedcontinuous annealing line, or on a continuous electrolyte tin line.

It is, therefore, an object of the present invention to provide animproved method of treating ferrous metal surfaces to produce surfaceswhich have high resistance to corrosion on exposure to atmosphericmoisture.

It is a further object of the invention to provide an inexpensive andrapid method of treating a ferrous metal surface to increase theresistance thereof to corrosion on exposure to atmospheric moisture.

It is still another object of the invention to provide an improvedmethod of treating ferrous metal surfaces to increase the corrosionresistance thereof which will also produce a surface having highlysatisfactory lacquering properties and having under-filmcorrosion-resistance.

It is also an object of the present invention to provide an improvedprocess for treating a surface of black plate to improvecorrosion-resistance thereof without materially discoloring the treatedmetal surface.

It is a still further object of the present invention to provide animproved process of treating black plate continuously and rapidly toincrease the resistance thereof to corrosion on exposure to atmosphericmoisture.

Other objects of the present invention will be apparent to those skilledin the art, from the detailed description and claims to follow.

It has been discovered that the foregoing and other objects of thepresent invention are readily achieved by immersing a metal surface fora very short time in a heated aqueous solution having a lowconcentration of sodium nitrite and ammonium molybdate. An unusuallyhigh degree of resistance to corrosion by atmospheric moisture isimparted to a ferrous metal surface by immersing said surface in orotherwise completely wetting the metal surface with an aqueous solutioncontaining about one-half of one percent by weight of sodium nitrite andabout the same amount of ammonium molybdate, each of the foregoing saltsbeing present in substantially equal amounts to provide an aqueoussolution having a slightly acid pH. The temperature of the aqueoussolution at the time of immersion should be from about F. to about 200F., and the duration of immersion of the metal in the solution, or theperiod during which the hot solution is maintained in contact with themetal surface should be between about 2 and about 4 seconds.

It is well known, of course, that both sodium nitrite and ammoniummolybdate have been used heretofore for treating metal surfaces toimpart increased corrosion resistance. However, neither one of thesesalts by itself is very effective for improving corrosion resistancewhen the duration of the treatment therewith is extremely short. Thus,when sodium nitrite is used alone, the concentration thereof Which mustbe used to obtain any appreciable protection must be relatively high andthe time of treatment prolonged. In addition, it is necessary to allowconsiderable nitrite salt to remain on the treated surface in order toobtain good corrosion resistance. When ammonium mo-lybdate is used aloneit is only partially effective and has the further great disadvantage ofdiscoloring the steel surface treated therewith.

The unexpected and unusual corrosion resistance provided by the presentprocess is obtained only when both the strength of the treating solutionand the duration of the treatment with the herein disclosed solution aremaintained within the narrow limits above specified. Thus, for example,if the concentration of the said salts in the solution and the durationof treatment exceed the specified limits, or if the combination of theconcentration of the salt and the time of treatment is excessive, theappearance of the sheet is impaired by retention on the surface of thetreated metal of insoluble iron bearing reaction products. For example,a solution containing a mixture of one percent by weight of each of thesaid nitrite and molybdate salts is entirely unsatisfactory even forshout-time immersion. It will thus be evident that in the presentinvention both the strength of the salt solution and the duration of thetreatment are critical and must be maintained within specified limits inorder ,to obtain acceptable results.

It is also important in order to achieve the desired optimum improvementin resistance to corrosion by atmospheric moisture that the surfacetreated be free of oil, grease, and other contamination, and in additionbe free of surface oxidation. To insure a satisfactory clean surface, itis preferable that the ferrous metal surface be first treated by briefiyimmersing or otherwise treating with a dilute mineral acid solution,such as with a dilute sulphuric acid pickle bath, for 1 to 2 seconds,followed by a clear water rinse. There should be no inhibitor in thepickle bath for best results. Other methods of cleaning the surface ofthe ferrous metal strip can be used, if desired, such as by means of analkali, bufling, or the like. Also, oxides on the metal surface can beremoved by pretreating with reducing gases, such as hydrogen.

The process of the present invention is particularly suited for thetreatment of elongated strips of fully finished ferrous metals, such asfull-finish black plate, which are processed by continuous annealing,followed by continuous skin rolling, and thereafter formed into a coilfor shipment. The processing steps required by the present invention canbe incorporated into the standard black plate production line without inany way interfering with the conventional steps thereof. Thus, as thestrip of black plates leaves the skin rolling station and beforecoiling, the strip can be continuously passed through a brief acidpickle bath in which it remains for a period of 1 to 2 seconds to cleanthe surface of soil and rust, followed by a clear water rinse, and thenis passed continuously through the heated nitrite-molybdate bathcontaining 0.5% by weight sodium nitrite and 0.5%

by weight ammonium molybdate in which the strip remains for a period of2 to 4 seconds. The treated strip is given a water rinse on emergingfrom the treating bath and is dried before coiling. The foregoingtreatment is carried out without in any way reducing or otherwiseinterfering with the high speed movement of the metal strip and producesa black plate having high resistance to corrosion.

In preparing the treating solution of the present invention, it isdesirable to first add suflicient ammonium molybdate to the desiredvolume of water to be used in the treating bath so that the molybdatesalt comprises 0.5% by Weight of the solution. The solution is thenheated to about 180 F. and to this heated solution is added suflicientsodium nitrite to provide a concentration of 0.5% by weight of the saidnitrite salt. Since the ammonium molybdate salt causes the solution tobe slightly acidic, said solution exhibiting a pH of about 4.85, nitrousacid is formed in the solution. The nitrous acid is unstable in the hotsolution, however, and gradually decompses to yield free oxygen with theoxygen appearing to film the ferrous metal surface. It also appears thatcertain iron molybdate salts are formed and react with the metalsurface. In view of the slow decomposition of the salts it is necessaryto replenish the salts from time to time in order to maintain theconcentration thereof at the herein specified levels. The solution ofthe salt as herein prepared, however, can be used several hours beforeit becomes necessary to replenish the said salts.

While it is not essential to the successful practice of the presentinvention to rinse the treated surface with water as it leaves thetreating bath when the nitritemolybdate solution has been freshlyprepared, it does become necessary to rinse the treated metal surfaceafter the treating solution has been used for some time, since looselyadhering brownish particles produced during the treatment tend to adhereto the surface of the treated metal. The use of a rubber squeegee roll,or the like, on the surface of the treated metal as the metal stripleaves the treating bath and before the water rinse also helps toprevent adherence of the reaction products to the surface of the metalstrip.

The protective film produced on the steel or other ferrous metal surfaceis of a light color and is hardly perceptible to the observer. It willwithstand the abrasive action without the film being damaged. Steelsheets which have been cleaned and treated in accordance with thepresent invention can be liberally sprinkled with water and allowed tostand in the air until dry without causing any rusting of the sheet. Thesheets treated in accordance with the present invention can alsowithstand a standard humidity cabinet treatment (high humidity at 100F.) for 48 hours with only about 5% of the surface rusting whereas anuntreated sheet will have 100% of the surface rusted within three hoursor less. Tests of the surface of steel panels treated in accordance withthe present invention have shown that substantially perfect adherence ofthe phenolic-type lacquers is obtained. In the later tests, the panelsurfaces were not rinsed following the treatment thereof so as todeliberately leave a residue of the treating bath on the surface todetermine if there was any adverse effect of a residue on lacqueradherence.

While the preferred embodiment of the present invention uses only sodiumnitrite and ammonium molybdate salts in substantially equal amounts sothat a slightly acidic solution is produced, it should be understoodthat other water soluble nitrite salts and other water soluble compoundsyielding molybdic acid in a heated aqueous solution can be used withoutdeparting from the scope of the present invention. Among the watersoluble salts and compounds which can be used in place of sodium nitriteand ammonium molybdate are: potassium nitrite, lithium nitrite, ammoniumnitrite, calcium nitrite, barium nitrite and strontium nitrite, andsodium molybdate, potassium molybdate and strontium molybdate,respectively.

It will also be understood that while the description has referred tothe ferrous metal being dipped or immersed in the nitrite-molybdatetreating solution, the solution can also be applied to the metal invarious other ways, including brushing, flow-coating, and spraying.

Others may practice the invention in any of the numerous ways which aresuggested to one skilled in the art, by this disclosure, and all suchpractice of invention are considered to be a part hereof which fallwithin the scope of the appended claims.

I claim:

1. A method of treating a ferrous metal surface to increase theresistance thereof to corrosion, comprising, contacting a surface of aferrous metal with an aqueous solution containing about 0.5% by weightof a water soluble nitrite salt and about 0.5% by weight of a watersoluble compound yielding molybdic acid in said solution whilemaintaining the said solution at a temperature between about F. andabout 200 F. and maintaining the solution in contact with the metalsurface for a period of about 2 to 4 seconds, and thereafter drying thetreated metal surface; whereby the metal surface is provided with a thinprotective film which is highly resistant to corrosion.

2. A method of treating a ferrous metal surface as in claim 1, whereinthe said nitrite salt is sodium nitrite.

3. A method of treating a ferrous metal surface as in claim 1, whereinthe said water soluble compound is ammonium molybdate.

4. A method of treating a ferrous metal surface as in claim 1, whereinsaid aqueous solution has a molybdic acid content sufficient to maintainthe pH thereof at about pH 4.85.

5. A method of treating a ferrous metal surface as in claim 1, whereinthe said ferrous metal comprises a strip of black plate.

6. An improved corrosion resistant ferrous metal product preparedaccordin to the process of claim 1.

7. A method of continuously treating a strip of black plate to increasethe corrosion resistance thereof which comprises, continuously passingblack plate through a dilute sulphuric acid pickle bath and allowing thesaid plate to remain in said bath for a period of about 1 to 2 secondsfollowed by a clear water rinse to clean the surface of said blackplate, continuously passing the clean black plate through an aqueoussolution containing 0.5% by weight of sodium nitrite and 0.5% by weightammonium molybdate and maintaining said black plate in contact with saidsolution for a period of 2 to 4 seconds while holding the temperature ofthe said solution between 180 F. and 200 F., rinsing the said blackplate with water to remove residual solution therefrom, and thereafterdrying the said black plate; whereby the surface of the black plate isprovided with a thin light-colored protective film which is highlyresistant to corrosion.

References Cited in the file of this patent UNITED STATES PATENTS2,557,509 Miller June 19, 1951 2,839,439 Stapleton June 17, 19582,854,370 Kronstein Sept. 30, 1958

1. A METHOD OF TREATING A FERROUS METAL SURFACE TO INCREASZE THERESISTANCE THEREOF TO CORROSION, COMPRISING CONTACTING A SURFACE OF AFERROUS METAL WITH AN AQUEOUS SOLUTION CONTAINING ABOUT 0.5% BY WEIGHTOF A WATER SOLUBLE NITRITE SALT AND ABOUT 0.5% BY WEIGHT OF A WATERSOLUBLE COMPOUND YIELDING MOLYBDIC ACID IN SAID SOLUTION WHILEMAINTAINING SAID SOLUTION AT A TEMPERATURE BETWEEN ABOUT 180*F. ANDABOUT 200*F. AND MAINTAINING THE SOLUTION IN CONTACT WITH THE METALSURFACE FOR A PERIOD OF ABOUT 2 TO 4 SECONDS, AND THEREAFTER DRYING THETREATED METAL SURFACE; WHEREBY THE METAL SURFACE IS PROVIDED WITH A THINPROTECTIVE FILM WHICH IS HIGHLY RESISTANT TO CORROSION.